What Is Power System Interconnection?

Power system interconnection is the physical and operational linking of separately owned or operated electric power networks into a larger, coordinated system that can exchange energy across shared transmission facilities. Interconnections range in scale from the tie lines between two neighboring utilities to the continental-scale synchronous zones that span multiple countries. By pooling generation resources and sharing transmission paths, interconnected systems achieve economies of scale, improve reliability through mutual support during outages, and enable trade in wholesale electricity across regional boundaries. The discipline of interconnection encompasses the engineering of the physical transmission links, the standards governing their performance, and the operating agreements that define how power flows are scheduled, priced, and controlled.

Interconnection engineering draws on transmission network theory, power system stability analysis, protective relaying, and the economics of long-distance power transfer. The technical requirements for interconnecting a new resource to an existing network are specified through interconnection studies that assess thermal, voltage, and stability impacts.

Grid Interconnections and Topology

The contiguous United States operates as three largely separate synchronous AC interconnections: the Eastern Interconnection, which covers the area east of the Rocky Mountains; the Western Interconnection, which extends from the Rockies to the Pacific coast; and the Electric Reliability Council of Texas (ERCOT), which covers most of Texas. As documented by the U.S. Energy Information Administration, these interconnections operate at the same nominal frequency but with separate phase angles and are coupled only through high-voltage direct current (HVDC) ties and variable-frequency transformers that decouple their AC frequency. Within each interconnection, dozens of balancing authorities manage the moment-to-moment balance of generation and load within their footprints.

Transmission Infrastructure

The physical backbone of an interconnection consists of high-voltage alternating current (HVAC) transmission lines operating at voltages from 115 kV to 765 kV, along with the substations, transformers, and switching equipment that form nodes in the network. Power flows on AC interconnections obey Kirchhoff's laws, distributing themselves across all parallel paths according to network impedances rather than following contracted schedules directly, a behavior known as loop flow. HVDC transmission systems, which convert AC to DC for long-distance transport and reconvert at the receiving end, offer controllable power flow unaffected by AC system impedance and are used for submarine cables, very long terrestrial links, and cross-interconnection ties. The FERC National Transmission Grid Study examines the role of new transmission investment in enabling renewable energy integration across existing interconnection boundaries.

Reliability and Operating Coordination

The reliability of a synchronous interconnection depends on the coordinated operation of all participants according to common reliability standards. In North America, the North American Electric Reliability Corporation (NERC) develops and enforces mandatory reliability standards covering planning, operations, and cybersecurity for the bulk power system. Transmission planning studies assess whether the interconnection can survive the loss of any single element, the N-1 criterion, without violating voltage, thermal, or stability limits. Interconnection agreements between utilities and new generators specify the technical requirements, cost responsibilities, and operating procedures that govern the physical connection. The FERC reliability explainer outlines how federal and regional bodies share responsibility for maintaining reliability across interconnected North American grids.

Applications

Power system interconnection has applications across utility planning, regulatory policy, and infrastructure development, including:

• Generator interconnection studies for new wind, solar, and storage projects
• Cross-border power trading between utilities in different reliability regions
• HVDC link projects enabling long-distance transfer between Eastern and Western Interconnections
• Transmission expansion planning to relieve congestion and improve access to remote renewables
• Emergency mutual assistance between neighboring utilities following large-scale outages